Method of manufacturing self-stressed concrete pipe



July 21, 1970 M. s. KRESTON METHOD OF MANUFACTURING SELF-STRESSEDCONCRETE PlPE Filed Aug. 31, 1967 mm: S

I NVEN TOR.

United States Patent 3,520,968 METHOD OF MANUFACTURING SELF-STRESSEDCONCRETE PIPE Max S. Kreston, Retllands, Calif., assignor t0 StressedPipe Research, Ltd., Littleton, Col0., a limited partnership of TexasFiled Aug. 31, 1967, Ser. N0. 664,701 Int. Cl. B28b 9/04 US. Cl. 264-2283 Claims ABSTRACT OF THE DISCLOSURE A method of manufacturing concretepipe comprising expansible concrete and internally, constrainingreinforcing in which the pipe is initially cast in a conventional mannerwithin a form; then, prior to the major growth period of the concrete,the form is removed and a longitudinal constraint is applied externallyof the concrete to supplement the longitudinal, internal constraintafforded by the reinforcing; then, after the growth cycle issubstantially completed, the external constraint is removed.

RELATED INVENTIONS This invention is related to the copendingapplication of Edward K. Rice and William C. Cureton, entitled: Self-Stressed Concrete Pipe and Method of Manufacture, Ser. No. 453,705,filed May 6, 1965.

BACKGROUND OF THE INVENTION This invention utilizes an expansiveconcrete. Expansive concrete contains ingredients which cause theconcrete to expand or grow during a critical period ranging from a fewhours to several days. The amount of expansion may be merely enough tocompensate for the shrinkage that normally occurs, or may besubstantially greater; in fact, the expansion may be so excessive thatunless the concrete is constrained during a critical period of itsgrowth cycle, the concrete will disintegrate.

Examples of cement formulations which produce expansive concrete of thetype suitable for use with the present method are found in Pats.3,155,526; 3,251,701; and 3,303,037 issued to Alexander Klein. It hasbeen found, for example, by tests conducted by T. Y. Lin and A. Kleinand reported in the A.C.I. Journal, Proceedings vol. 60, No. 9,September 1963, pages 1187-1218; that concrete bodies of simple shapecan be formed with highly expansive concrete if the concrete is castabout reinforcing steel, and the concrete is properly bonded to thesteel before appreciable expansion takes place. The resulting concretebody is held in compression by the reinforcing and the reinforcing isplaced under tension.

This type of product has been referred to as chemically stressed orself-stressed concrete, as distinguished from mechanically pretensionedor posttensioned concrete. It is well known that concrete has extremelyhigh strength in compression, but no appreciable strength in tension.This has brought about the use of pretensioned and posttensionedstructures which are so designed that under the intended loads, theconcrete is held under compression by highly stressed steel tendons.

Previous use of posttensioned and pretensioned concrete pipe hasestablished the advantages of such pipe or other pipe materials to carryor contain fluids under pressure; however, the cost thereof has beenexcessive. In theory, a self-stressed concrete pipe should have theadvantages of conventionally stressed concrete pipe, yet previousattempts to produce a commercially acceptable self-stressed concretepipe have not been successful due to the difficulties encountered inobtaining proper conice straint throughout the concrete pipe, especiallyat the ends thereof.

SUMMARY OF INVENTION The present invention is directed to a method ofmanufacturing self-stressed concrete pipe and included in the objects ofthe invention are:

First, to provide a method of manufacturing selfstressed concrete pipewhereby the concrete, including the concrete at the ends of the pipe isplaced under compression and held so by the internal reinforcing so thatthe resulting pipe does not delaminate, disintegrate, or develop crackseven though subject to internal pressures of a magnitude tending toneutralize the compression loads within the concrete.

Second, to provide a method of manufacturing selfstressed concrete pipewhich can be employed in combination with, or as a supplement to, mostof the conventional processes now used in the manufacture of concretepipe of all sizes; for example, but not limited to centrifugal casting,vertical casting using inner and outer forms, and packer head or tampmachines, or any repetitive process wherein concrete is cast in formsand the forms are removed and reused.

Third, to provide a method of manufacturing selfstressed concrete pipe,whereby axial expansion of the pipe is controlled not only by thepermanent internal reinforcing within the walls of the pipe, but also bytemporary external restraint, which may be, but is not limited to,threaded steel rods with locking nuts.

Fourth, to provide a method of manufacturing selfstressed concrete pipewhich by selection of the amount and arrangement of internal reinforcingand temporary external restraint, the strength of the pipe may be variedto meet a wide range of combined internal pressure loads, earth loadsand superimposed live loads.

Fifth, to provide a method of manufacturing selfstressed concrete pipewhich adds only a minimal cost to the method of making conventional pipeyet materially increases the strength of the concrete pipe so that theself-stressed concrete pipe may be used in many installations wherepreviously only steel pipe could be used.

DESCRIPTION OF FIGURES FIG. 1 is a fragmentary sectional view of acentrifugal casting mold with a completed pipe therein.

FIG. 2 is a similar fragmentary sectional view showing the concrete pipeafter removal from the mold and provided with end and longitudinalconstraints to control tensioning of the reinforcing during theexpansion cycle of the concrete.

FIG. 3 is a partial end, partial sectional view of the concrete pipe inthe condition shown in FIG. 2.

SPECIFICATION The method of manufacturing self-stressed reinforcedconcrete pipe may utilize any of the conventional apparatus employed tomold or form the pipe. For purposes of illustration, a centrifugalcasting mold is shown. This conventional type of mold includes an outerform 1, comprising two or more sections which are joined by bolts 2,extending through flanges 3 at the confronting margins of the formsections. If the pipe to be cast is of the bell and spigot type, theform is enlarged at one end as indicated by 4 to cast the bell end ofthe pipe. If the pipe is of the double spigot type, then both ends ofthe mold are identical.

In order to contain the concrete, the outer form is provided with aspigot forming band 5 and a spigot ring 6. In conventional practice, theband 5 and ring 6 may be integral, but in the exercise of the presentinvention, it is desirable that they be separated. Suitably cemented orotherwise held in place within the band 5, is a band 7 which forms theseal ring groove.

A bell end ring 8 is fitted within the enlarged or bell end 4 of theouter form. The bell end ring may be conventional and includes anoutwardly directed flange 9 joined to a bell forming band 10, which inturn is joined to an internal flange 11, offset from the flange 9.

As in conventional practice, prior to casting the pipe, a reinforcingcage is placed in the form. The reinforcing cage includes longitudinalreinforcing members 12, and circumferentially wrapped reinforcing 13.

In the casting of self-stressed concrete pipe, expansive concrete issubstituted for conventional concrete. The term expansive concrete, asherein used, refers to concrete containing a portland-type cement towhich has been added ingredients which cause the cement and the concretecontaining the cement to expand in an amount sufficient to causereinforcing contained in the concrete to be placed under tension. Thatis, the reinforcing prevents the concrete from expanding freely with theresult that after the expansion cycle, the reinforcing is under tensionand the concrete is under compression.

The expansion cycle actually starts upon mixing the concrete; however,the effective exansion begins as the concrete undergoes initial set andbegins to bond to the reinforcing during a period beginning a few hoursafter the concrete has been mixed and continues for about twenty-four toseventy-two hours thereafter, or longer by selection of the variousformulas as described in the aforementioned Klein patents. It ispossible by controlling the percentages of the various ingredients topredetermine the duration of the expansion cycle and the amount ofexpansion within fairly close limits and also to control othercharacteristics of the concrete mixture. For example, it is possible toeffect early bonding of the concrete to the reinforcing steel so thatthe steel may act as a constraint early in the expansion cycle.

Expansive concrete of the type suitable in the practice of the presentinvention, has been fully set forth in the hereinbefore mentioned Kleinpatents.

In the practice of the present invention, the concrete pipe, designated14,- is removed from the form 1 after it has set sufficiently to beself-supporting, but before any significant expansion has occurred. Theband is removed, but the ring 6 is retained as well as the bell end ring8. The pipe thus removed includes a spigot end 15, having a gasketrecess 16 and a bell end 17, having a bell cavity 18. However, the pipemay be of the double spigot type.

Before or at the beginning of the expansion of growth cycle, endrestraining rings 19 are placed over the ring 6 and the internal flange11 of the bell end ring 8, as shown in FIG. 2. The end restraining ringsare joined by tie rods 20 which are secured by nuts 21. The rings 19 maybe annular or may be solid plates, also these rings may be integral withthe ring 6 and the bell end ring 8. The tie rods may be initiallyadjusted so that initially they are free of tension or may be tensionedto some selected amount by a torque wrench.

The number and diameter of the tie rods may vary depending upon the sizeof the pipe and also may vary depending upon the internal pressure forwhich the pipe is designed.

In some instances, the end restraining rings and tie rods may be mountedbefore the pipe is removed from the outer form 1. It is essential,however, that the pipe be removed from the outer form before the formacts as a significant constraint against radial exansion of the concretefor the reason that it is intended to place the circumferentialreinforcing 13 under tension. That is, the the circumferentialreinforcing is preferably the sole radial constraining means. This doesnot preclude the use of an outer form as a supplementary constraintproviding that the form is capable of expanding in response to theforces generated within the concrete to permit proper tensioning of thecircumferential reinforcing 13.

Usually, however, ample constraint may be obtained from the reinforcing13.

Longitudinal constraint of the concrete pipe poses a more difficultproblem. In the case of the circumferential reinforcing, the bondbetween the wire and the concrete is subject to loads which istransverse to the wire, whereas the bond between the longitudinalreinforcing and the con crete is longitudinally of the wire, that is,the bond is subjected to shearing forces and is of course, much lessresistant to shearing forces than to forces applied perpendicular to thereinforcing. As a consequence, the longitudinal reinforcing, even thoughwrapped with circumferential reinforcing, is not adequate to hold theconcrete so that the bond is destroyed or at least damaged so thatcontrol over the expansion of the concrete at the ends of the pipe islost or impaired and the ends of the pipe are weakened.

By supplementing the longitudinal constraining effect of thelongitudinal reinforcing 12 with the constraint afforded by the tie rods20 and further by distributing the load over the ends of the concretepipe by means of the rings 19, the spigot end ring 6, and the bell endring 8 during the growth or expansion cycle, the proper bond ismaintained and the optimum longitudinal compressive stress is setup inthe concrete.

As indicated previously, the present method may be added to orsupplement various methods of molding concrete pipe. If a verticalmethod is used, employing inside as well as outside forms, at least theoutside form need be removed. The inside form may remain intact, as theconcrete pipe expands away from the inside form. If the inside formremains intact, the tie bolts or tension bolts 20 may be placed in aring outside the pipe rather than within the pipe. If a packer headmethod of forming pipe is used, no inside form is required so thatcontrol of the compression stresses in the concrete and tension stressesin the reinforcing may proceed in the same manner as that described inconnection with the centrifugal casting of the concrete pipe.

While particular embodiments of this invention have been shown anddescribed, it is not intended to limit the same to the details of theconstructions set forth, but instead, the invention embraces suchchanges, modifications and equivalents of the various parts and theirrelationships as come within the purview of the appended claims.

I claim:

1. A method of manufacturing self-stressed reinforced concrete pipeutilizing a concrete, which, after being cast and set to self-sustainingcondition, undergoes a chemical expansion cycle and, early in theexpansion cycle, bonds to reinforcing contained therein; then, duringfurther expansion, places the reinforcing under tension and the concreteunder compression, said method characterized by:

(a) introducing said expansive concrete into a mold which contains areinforcing cage having circumferential and longitudinally extendingmembers, and

fully embedding the cage in the concrete;

(b) placing restraining end elements in external abutment with the endsof said cast pipe at the beginning of said effective expansion cycle;

(c) connecting said end elements by tension members, externally of saidconcrete, to restrain longitudinal expansion;

(d) removing the concrete body from the mold early in the expansioncycle and after it has set to selfsustaining condition;

(e) restraining the concrete solely by the internal reinforcing, andelements and tension members during the remainder of the effectiveexpansion cycle;

6 (f) and removing said end elements and tension mem- (d) and removingsaid end constraints and tension elebers at the end of said effectiveexpansion cycle. ments at the end of said effective expansion cycle. 2.A method, as defined in claim 1, wherein: (a) said tension members areplaced within the bore References Cited 3 y i p p f hf d t 5 UNITEDSTATES PATENTS n e manu ac ure 0 rel orce concre e plpe, wherein thepipe is initially formed by a conventional gfi method in which areinforcing cage is fully embedded in 2474660 6/1949 i i 264 42 thecodngrete, the method of self-stressing the pipe charac- 27O9845 6/1955ig: 56 X terize y:

(a) employing a concrete, which, after being cast and 10 329084O 12/1966Mlddendorf 264228 X set to self-sustaining condition, undergoes a chemi-FOREIGN PATENTS cal expansion cycle and, early in the expansion cycle,559,213 6/1958 canada bonds to reinforcing contained therein; then,during further expansion, places the reinforcing under ten- 15 OTHERREFERENCES sion and the concrete under compression; ChemicalPrestressing of Concrete Elements Using (b) supplementing thelongitudinal restraint produced Expanding Cements, Lin, T. Y. and Klein,A., Journal of by said reinforcing cage by placing end constraints theAmerican Concrete Institute, September 1963, pp. at and in externalabutment with the extremities of 1200-1203. the pipe and connecting theend constraints by exposed tension elements extending between the endsROBERT F. WHITE, Primary Examiner of Sald P e; R. R. KUCIA, AssistantExaminer (c) exposing the formed concrete pipe to expansion during themajor portion of its expansion cycle ex- S. C X.R.

cept as restrained by said reinforcing cage, end con- 264 229 231straints and tension elements; i i i

